eburacum wrote:I tend to subscribe to the idea that time travel alter history and would lead to the creation of a new, divergent universe,
and so you cannot travel to your own past.
The moment you set foot in the past you create a new future which is different from the one you left.
My time travel rant is here;
http://www.orionsarm.com/intro/WhyNoTimeTravel.html
I agree with this view point, which is compatible with the Everett interpretation of Quantum Mechanics.
There are many things we could say about time travel. According to Stephen Hawking, a time machine isn't able to travel before its own construction. So you wont be able to visit Newton to kill him. However, if a time machine was build before your own birth, you could travel to a moment in time before your own creation and kill your mother. Then a paradox.
In classical general relativity, there are many solutions which describes space-time with closed time-like curves (Kerr-Newman solution, Godel universe, and worm holes are well known examples), which means you could use them to travel in the past.
But there is a Fondamental principle in physics : consistency of Nature. There is no paradoxes in Nature, whatever you're doing. Coherence of a theory is the most important thing. Because of this, relativity theory implies THERE IS NO FREE-WILL IN ANY CLASSICAL SPACE-TIME ! So you can't decide to travel in your own past to kill your mother. Even if a time machine was possible, you can't use it as you wish, because you don't really have a free will. Let me explain this.
In classical relativity, space-time is the set of all events, in space AND in time. All events are actually lying there, "somewhere", and they all "coexist" already. Everything is already "writen" in space-time. Relativity theory, when placed in a fully classical context, is deterministic and FATALIST. You can't change any event, because all events are already "memorised" in space-time. There's no place at all for free-will. So even if time travel is possible, you can't change anything. There is no paradox at all. Only coherence of space-time count here.
But if you really believe in free will, and aparently we have access to free will in our real universe, then the space-time of relativity theory must be demultiplied in some infinite set of space-times with many variations (the Multiverse). All those space-times are deterministic and fully fatalist, but you don't know in which space-time you are living. They becomes probabilistic. Your future isn't really "writen" somewhere, because now we have an infinite set of space-times.
Actually, space-time in classical relativity theory is just a usefull concept, a tool, to describe reality. But space-time isn't "real". It's just a representation, a partial map of events. The real space-time doesn't even exist (in the sense of relativity theory). There is no future "coexisting" with the past, and the past itself is disapearing at all moments.
"Well! I've often seen a cat without a grin", thought Alice; "but a grin without a cat! It's the most curious thing I ever saw in all my life!"
) I kill him...
